Cast nickel-base alloy

ABSTRACT

A cast nickel-base alloy having outstanding utility in very high temperature application at a composition consisting essentially of, by weight, 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, up to 0.05 percent carbon, balance nickel.

United States Patent [15] 3,655,462 Maxwell [45] Apr. 11, 1972 [5 CAST NICKEL-BASE ALLOY [56] References Cited [72] Inventor: Douglas H. Maxwell, Monsey, N.Y. UNITED STATES PATENTS 1 Assignee: United Aircraft Corporation, East Hart- 2,542,962 2/1951 Kinsey ..75/l70 ford, Conn. I 22 Filed: Man 22 1971 Primary Examiner-Richard 0. Dean Attorney-Richard N. James [21] App1.No.: 126,802

57 A TRACT Related US. Application Data 1 1 k lb 11 h d T h h cast me easea 0y avmg outstan mg uti ityin very lg Dmslon ofser- 800,591 1969- temperature application at a composition consisting essentially of, by weight, 17.5-18.5 percent molybdenum, 1. .5. er e t aluminum up to percent carbon [51] Int. Cl ..C22c 19/00 balance nicke] [58] Field of Search ..75/170, 171; 148/32, 32.5

2 Claims, 3 Drawing Figures &/' Ma-flZ 1 o- ZWZ AZ PATENTEDAPR 1 1 m2 SHEET 2 [IF 3 CAST NICKEL-BASE ALLOY BACKGROUND OF THE INVENTION This is a division of application, Ser. No. 800,591, filed Feb. 5

The present invention relates in general to the cast nickelbase alloys, particularly those adapted to high temperature service.

In the design of the more advanced gas turbine engines one of the principal limitations imposed on the designer in terms of providing increased engine performance and efficiency are the high temperature strength limitations of the available engine alloys. Even in todays operational engines, the various alloys utilized are frequently exposed under high stress to temperatures in excess of 85 percent of their melting points. Accordingly, an urgent need exists for new materials which will not only provide improved high temperature strength in engine environments, but which will also display such characteristics as oxidation, fatigue and creep resistance.

In the prior art US. Pat. to Kinsey No. 2,542,962, there is described a series of alloys within the broad compositional range of, by weight, -35 percent molybdenum, tantalum, tungsten and/or columbium, 2.412.l percent aluminum, up to 0.15 percent carbon, balance essentially nickel. It has now been discovered that, within the broad range disclosed-by Kinsey, there exists a unique alloy displaying useful properties unexpectedly superior to those of the currently available alloys. As hereinafter discussed in detail, the compositional limits of this new alloy are so critical that any significant variation therefrom results in a totally unsatisfactory material for high temperature use. Indeed, because of the narrowness of the compositional limits involved in this new alloy, it is not surprising that its remarkable properties have not previously been discovered. Despite a similarity in overall chemistry to the prior art alloys, it is only within the narrow compositional limits disclosed herein that the particular advantageous metallurgical structure is achieved as clearly evidenced not only by the startling differences in mechanical properties of this alloy but also by its metallurgical uniqueness as evidenced by metallographic examination.

SUMMARY OF THE INVENTION The present invention contemplates a cast alloy at the basic composition of, by weight, 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, up to 0.05 percent carbon, balance nickel. In this alloy tantalum may be included on a substitutional basis for the molybdenum in an atomic ratio of up to about 1/ 1. Similarly, tungsten may be included in this alloy on a substitutional basis for the molybdenum up to about 16 weight percent tungsten based on the total weight of the constituents, although tungsten drastically decreases the fabricability of the alloy and increases its specific weight and, thus, in many instances is not preferred. The total tantalum and tungsten substitutions, if made, should be limited to about 25 weight percent of the total weight. Still further, the alloy is receptive to the addition of reactive metals, such as yttrium, scandium and lanthanum, which are often utilized to foster oxidation-erosion resistance, in weight percentages up to about 1 percent.

In a particular preferred embodiment of the invention as conventionally cast yielding either a substantially equiaxed grain structure, or castings in columnar-grain or monocrystal form the composition comprises, by weight, 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, 0.03-0.05 percent carbon, up to 0.20 percent manganese, up to 0.20 percent silicon, up to 0.015 percent sulfur, up to 0.35 percent iron, up to 0.10 percent copper, balance essentially nickel.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph illustrating the stress-rupture capability of the cast alloys of the nickel-molybdenum-aluminum system, at 1,800 F. and 15,000 p.s.i., as a function of their molybdenum and aluminum content and relationship.

FIG. 2 is a graph comparing the alloy of the present invention with the conventional MAR M200 alloy, as directionally solidified, in creep at various temperatures.

FIG. 3 dramatically illustrates the effect of directional solidification on the 2,200 F. creep of the nickel-molybdenum-aluminum alloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As hereinafter used in the description, the alloys of the present invention are often referred to as the Nil8Mo 8A1 alloys.

At a nominal composition of 18 weight percent molybdenum, 8 weight percent aluminum, balance essentially nickel, and within the ranges of 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, up to 0.05 percent carbon, balance essentially nickel, the alloys of this invention comprise a dispersion of the strengthening y nickel aluminide precipitate, Ni Al, at about volume percent, in a solid solution of nickel and molybdenum.

The extreme criticality and interrelation of the molybdenum and aluminum components of this alloy are dramatically illustrated in FIG. 1, and still further in the following table.

As clearly evidenced by the data, the strength of the present alloy is profoundly influenced by the amount of molybdenum and aluminum present in the alloy particularly in combination. At low aluminum levels, the high temperature capabilities of the alloy are adversely affected with dramatic suddenness so that at 2,200 F. the alloy is completely lacking in any useable strength. This is unusual in the sense that a relative minor reduction of the low melting component drastically influences -its high temperature strength. Actual analysis of the phenomenon reveals that, at the lower aluminum levels the y precipitate is solutioned at lower temperatures.

At the higher aluminum contents, 9 percent by weight for example, the strength of the alloy is suddenly and markedly reduced, as compared to the alloy of the present invention, at all temperatures. The extent of the sudden strength decrease associated with such a small increase in aluminum content is believed to be a direct function of the conversion of at least a portion of the high strength y aluminide, Ni Al to the lower strength [3 aluminide, NiAl.

With respect to the molybdenum content of the alloy it is evident, particularly after reference to FIG. 1, that extreme criticality exists in terms of composition and particularly insofar as the molybdenum/aluminum relationship is concerned. At molybdenum contents lower than those associated with the present invention, there is insufficient molybdenum present for adequate solid solutioning strengthening. At the higher molybdenum contents the alloy is susceptible to the formation of brittle intermetallics which prove fatal to the alloy insofar as gas turbine engine applications are concerned.

Carbon, or the absence of carbon, in the present alloy also assumes a rather critical role, particularly in columnar-grain or monocrystal form. In those components formed by conventional casting techniques to provide substantially equiaxed microstructures, carbon in the range of about 0.03-0.05 weight percent is advantageously included to provide creep and tensile ductility. In the directionally solidified components, however, the carbon content is necessarily limited to minimize the occurence of the M C-type carbides. Thesecarbides, which assume considerable size in directional solidification casting because of the relatively slow solidification rates normally associated therewith, have been formed to be subject to precracking during solidification or to initiate failure sites for cracking during use. Inasmuch as in the columnar-grain or TABLE IV RT R Hardness after Heat Treatment for Alloys A, B, C and D C dt' 23W! monocrystal form of the alloy, gram boundanes are e1ther 5 Alloy g i H aligned in the direction of apphed stress or are virtually nonex- Ni- 1 2Al- 121140 36.0 35.0 36 1 istent, the presence of carbon for creep and tensile ductility is not re uired N1-8Al-l8Mo 37.8 37.0 39.0 q Ni-SAl-IZMo 27.2 211.0 34.0 In one senes of tests, 1nvolv1ng 24 d1rect1onally sol1d1fied bars in two heats, E3367 and F-3368, the following observa- TABLE V tions were made. The actual chemical analyses of these heats as determined by the suppher were as follows: Effect of Heat Treatment on Ni 18Mo 8A1 Heat A1(wt. Mo(wt.%) C(wt.7z) Ni 5 Tensile Properties 02ers 1115 v. F-3367 1.57 17.7 0.01 11111.

' Condition Temp, "F Ksi Ksi Elon F4368 As cast 1800 73.0 34.7 4.5 2250 F 4 00 11100 91.6 102.5 4.0 The d1fference 1n chemlstry between the two heats, 77" F/hr cool although slight, resulted in a difference in microstructure. 20 from Both alloy compositions contained a nickel eutectic-type Stress Ru mm Pro emes phase in a region of precipitate (Ni Al) plus a 7/ solid solup p tion (Ni,Mo,Al), the eutectic-type phase being identified by Test stress Hours a; electron m1croprobe analys1s as havmg a compos1t1on of Condition temp. F Ksi rupture Eton about, by weight, 39 percent molybdenum, 4 percent alu- 2 :83? 3-8 if}: r; minum, balance nickel. However, metallurgically speaking, g f 2000 there was a noticeable difference between the two heats as 230o|= 1 0 2000 8,0 31,5 2,3 evidenced by the almost complete absence of the eutecticc0001 type phase in the F-3368 (low aluminum) alloy. 2300 F 2000 275 73 it is, of course, evident from the data that the properties of TABLE V] these alloys are drasncally influenced by the alloy compositlon and within very precise compositional limits. This is the direct A C Impact D f r N14 g -g result of the fact that, despite somewhat closely, related chemical compositions, the solidified articles at the various 35 'Fhwpysmoolh C ry Iwwhed compositions are in fact distinctive entities metallurgically. 32: fg A number of alloy melts were made in a variety of composi- 5M 9'0 tions, the representative results of which are summarized '10 53.5 below 1800 110.0 9.0

TABLE IL-ALLOY CHEMISTRY Alloy A Alloy B Alloy 0 Alloy D Element, wt. percent Aim Actual Alrn Actual Alm Actual Aim Actual 12 11.1 10 9.3 s 7.9 s 7.7 18 1s. 1s 18 18.0 18 18. 0 12 12. 0 Balance Balance Balance Balance TABLE 111 [As Cast Mechanical Properties 01 Four Nl-Mo-Al Compositions] As Cast Tensile Properties As Cast Stress Rupture Properties R As cast Test 0.2% Test Hours hardtemp., YS, U'IS Percent temp., Stress. to Percent Alloy ness F. k.s.l. k.s.l. elon. F. k.s.l. rupt. elon.

Nl-12Al-1BMO 36. 0 70 85. 0 115. O 2. 0 1, 800 15. 0 8 9. 7 70 87. 7 116. 0 2.0 l, 000 112. 9 127. 6 2. 0 1, 400 86, 5 109. 0 2, 000 6.0 1, 800 57. 2 3.0 2, 000 26. 3 28. 1 5. 5

N i-10A1-18M0 32. 2 70 75. 3 123. 4 4. 0 1, 800 15. 0 6. 3

Nl-8Al-18Mo 37.8 70 102.0 163.6 3.5 1,400 85.0 10 3 2 4 70 102. 6 140. 2 8. O 1,000 121. 0 163.0 8. 5 1, 800 15. 0 118. 2 1, 400 111. 0 143. 9 3. 5 l, 800 15. 0 92. 5 1, 800 73. 0 84. 7 4. 5 2, 000 49. 0 68. 3 5. 0 2, 000 8. 0 28. 3 2, 000 56. 9 64. 3 5. 5 2, 000 8. O 37. 4 2, 200 16. 8 25. 4 15. 0 2, 200 3.0 16. 1

Nl-8Al-12M0 27. 2 70 79. 6 143. D 17. 5 1, 800 15. 0 16. 8

70 79. 6 136.0 15.5 1, 000 ill). 3 111. 8 5. 5 2, 000 8. 0 7. 2 1,400 105.5 108.8 1.0 1, mm (13.11 2, 200 1.0 10.11 2. mm 1:1. 1 4 1. 5 1 0 Alloy C exhibited the best overall properties with useful tensile and stress-rupture strengths to 2,200 F. Alloys A, B and D were not considered satisfactory due either to low rupture strength and/or ductility. Alloy C further showed a major response to homogenization heat treatment at 2,250-2,300 5 F. in that elevated temperature tensile strength was significantly improved over the as-cast properties. The stress-rupture properties on the other hand did not appear to be influenced by a high homogenization heat treatment. Alloy C further showed excellent potential for hardfacing applications since its hardness after a drastic cooling was observed to be significantly higher than its as-cast hardness. in summary, the results indicate that the Ni-l8Mo-8A1 alloy is unique, has outstanding tensile strength in the l400-2,200 F. range; and useful stress-rupture strength in the 2,000-2,200 F. range.

Testing of the alloy in the ascast or equiaxed condition revealed that both rupture strength and ductility were controlled primarily by the ability of the grain boundaries to resist fissuring and sliding. in the Nil8Mo8Al alloy only a fraction of the creep damage incurred during testing could be attributed to plastic deformation within the grain itself. Ac-

cordingly, although the alloy has superior properties in the equiaxed condition, the true strength of the alloy could not be approached until all grain boundaries not parallel to the stress axis. This was accomplished by directional solidification of the alloy to produce a columnar grain structure in which the existing grain boundaries were aligned with the axis of applied stress. In directional solidification testing performed with stock from the F-3367 and F-3368 heats previously mentioned, the effect of directional solidification of the 2,200 F. creep of these alloys was observed, as set forth graphically in FIG. 3.

Additionally, twelve test bars were directionally solidified at the following chemistry in two separate heats:

Element Percent by Weight A1 8.0 M0 18.26 C 0.01 S 0.003 Si 0.05 Mg 0.05 Fe 0.05 Co 0.05 Ni Bal.

This material, tested uncoated in air, displayed the tensile and creep i'upture properties reported in Tables VII and VIII.

TABLE VIII.-CREEP RUPTURE Hours Final Temg Stress, To elongation, p.s.i To 1% rupture percent Condition:

H. 1,400 85,000 52.3 1.9 H. 1, 800 15, 000 110 394. 7 14. 5 H. 1, 800 20. 000 41 105. 4 10. 5 H 1,800 I 25,000 5 21.8 11.4 H. 1, 900 22, 500 3 9.0 15. 6 H. 2, 000 9, 000 84 191.8 10.0 A. 2,000 13.000 5 35. 5 36. 2 H. 2, 000 13, 000 27 49. 3 l2. 6 H. 2, 100 8, 000 35 186. 5 8. 1 H. 2, 100 10, 000 43 107. 5 8. 5 H. 2, 200 5, 000 65 100. 5 11.0 2,200 6, 500 12 33. 7 1s. 4

sZL-T: isektis t aen (man) 2. 1 0? 1 ip-a 0 a s A comparative summary of various materials in several metallurgical forms is reported in the following table.

In a supplemental program, the Nil8Mo-8A1 was tested in a coated condition to determine if either the conditions under which the coating was applied or a subsequent diffusion of coating elements would have any deleterious effect on the alloy substrate. Coated specimens (chromium modified aluminide) tested at 1,800 F. and 2,000 F. revealed no detrimental effects from the coating utilized and, in fact, exhibited improved lifetimes.

In view of the foregoing and within the specific compositional limits set forth, the alloy of the present invention may be seen to possess a uniqueness which renders it particularly suitable for high temperature applications in the advanced gas turbine engines. its obvious superiority over the currently utilized alloys immediately suggests such use. However, it will be understood that its utility is not so limited, and its use in other applications such as hardfacing, or high temperature mold or die construction is suggested.

What is claimed is:

1. As an article of manufacture, a casting shaped to receive and define a material forced thereinto in a forging process comprising an alloy consisting essentially of, by weight, 17.5-18.5 percent molybdenum, 7.75-8.25 percent alu- .minum, up to about 0.05 percent carbon, up to about l percent yttrium, scandium or lanthanum, up to 25 percent tantalum plus tungsten on a substitutional basis for molybdenum, the tungsten content not exceeding about 16 percent based on the total weight of the alloy, the tantalum content not exceeding 50 atomic percent based on the molybdenum content of the alloy, balance nickel, the alloy being characterized by a volume percentage of the y nickel aluminide over percent, useable strength to at least 2,250 F. and a substantially equiaxed polycrystalline microstructure.

2. A casting according to claim 1 wherein the alloy consists essentially of, by weight, l7.5-l8.5 percent molybdenum, 7.75-8.25 percent aluminum, 0.03-0.05 percent carbon, balance nickel. 

2. A casting according to claim 1 wherein the alloy consists essentially of, by weight, 17.5-18.5 percent molybdenum, 7.75-8.25 percent aluminum, 0.03-0.05 percent carbon, balance nickel. 